Curious but interesting
I have been measuring and comparing the spare Nordie/RC barrel and piston I brought back with me with the ex 72degrees Nordie hillclimb Frigioli big bore barrel.
Nobody, including Frigioli, could remember, or tell me the head gasket thickness, so I assumed it must be the same 1.4/1.5mm compressed thickness of the standard gasket.
Putting the 2 barrels side by side the Nordie/RC base to head face is longer at 75.5mm than the Frigioli 74.2mm, implying that the Frigioli gasket would be 2.8mm thickness to maintain the timing belt relationship between the crank pulley and cam pulleys, alter this and you are playing with vernier pulleys to get the correct cam timing.
We know that 72degrees had gasket leaking issues with compression gasses pressurising the cooling system, I suspect this cannot have been helped by having such a thick gasket, the best leak-less joint is not to have one, or have lapped metal to metal faces as used in historic Napier engines, or gas filled sealing rings as used by NSU in some of their 1970’s car engines, never a gasket failure.
Frigioli machined a raised 0.5mm high step around the edge of the bore to increase sealing pressure onto the gasket at the head face, which implies they were trying to cure a known problem??
So gentlemen what are your thoughts on this, stay with the thick gasket, known to be problematic?
Increase the base thickness gasket and use a reduced thickness head gasket?
Increase the base thickness gasket more and go metal to metal using a fluorocarbon O ring seal and high temperature silicon sealer around the water passages?
Increase the base gasket and I'll machine you up copper head gaskets as promised - you'll only shift the problem by thickening up the base gasket considerably and possibly open up a can of worms with no head gasket at all.... At least try a compromise and if it still buggers up you can then look at alternatives. A 2.8mm head gasket seems just a bit thick....
But then what do I know, I spend an unhealthy amount of time tinkering with Husqvarna's...
The thought of that 2.8 gasket worries me, it has to be a week area prone to blowing. A thicker base gasket or shims (not a new idea, much used post war to alter compression in Manx Nortons etc)and using a 1.4-1.5 thick gasket would allow the Frigioli barrel to be tested without any machining, so O-B any time you get chance 2 gaskets please
I don't remember the head gasket being anything like that thick. In fact, after I blew it the first time, I had a solid copper gasket made up by someone (I forget where) that was more like the thickness you mention. There's a very remote chance I still have the 'blown' original and the 'pattern' in the garage - but probably thrown out in a clearout last year.
With the thick gasket it went well enough to suggest any cam timing side effect wasn't that disastrous. It also actually managed a couple of runs up the Lerghy Frissel 5 Km hill before it began to pressurise.
Then I rebuilt it again with what I think was one of Frigerio's last 'thin' to my eyes gaskets. After that it soon needed regular attention with a bicycle pump applied to the radiator expansion tank. One run up Gurston was enough
As far as I remember, Gary Rowe was thinking of trying an 'o' ring. I know he had his head and barrel surface ground and presumably a custiom gasket made in an attempt to cure it. His bike could survive at least a couple of laps of the TT course. Though the dreaded cracked crankcase got him anyway the first year (he had race cams and over 60 BHP).
"Do not adjust your mind, there is a fault in the reality" 1957 175 Sport